class Animal:
    def speak(self):
        print('····')


class Dog(Animal):
    def speak(self):
        print('汪汪汪~~~')


class Cat(Animal):
    def speak(self):
        print('喵喵喵~~~')


class Duck(Animal):
    def speak(self):
        print('嘎嘎嘎~~~')


dog = Dog()
cat = Cat()
duck = Duck()
dog.speak()
cat.speak()
duck.speak()
